The present study is designed to show that 5-hydroxytryptamine (5- HT) is used as a neurotransmitter in the leech. Recently completed work towards this end consists of 1) a description of the 5-HT containing terminals on the body wall muscle of the leech, 2) a count of the total number of axons in the peripheral nerves and connectives, combined with autoradiographic data on the number of 5-HT and catechol amine axons in these structures, 3) a description of the sheaths of the peripheral nerves in the body wall, 4) a chemical and autoradiographic analysis of the fate of exogenously administered 5-HT in this animal. These studies are all in press or being made ready for submission. Experiments are beginning on the injection of the 5-HT containing neurons with radioactive 5-HT and on stimulating these cells and collecting 5-HT from the body wall.